As the multilayer printed circuit board, there is a build-up multilayer printed circuit board in which a conductor circuit and a resin insulating layer are alternately built-up and an inner circuit layer and an outer circuit layer are electrically connected to each other through viaholes or the like. This build-up multilayer printed circuit board is generally manufactured by a method wherein a layer of the conductor circuit is formed in a position closest to a substrate by a subtractive process and then a plurality of conductor circuit layers are laminated on the conductor circuit layer by an additive process.
The subtractive process is a method of forming the conductor circuit by subjecting a surface of a copper lined laminate to an etching treatment and can cheaply form the conductor circuit layer having a high reliability.
The additive process is a method wherein an adhesive for electroless plating is applied onto a substrate such as glass epoxy or the like to form a resin insulating layer and then the resin insulating layer is roughened and a plating resist is formed on the roughened surface and then a metal for conductor circuit is applied thereto by electroless plating. According to this method, the conductor circuit is formed on the roughened resin insulating layer through the plating or the like, so that the excellent adhesion property therebetween can be ensured and hence there can be manufactured a printed circuit board in which the conductor circuit hardly peels from the resin insulating layer.
In the multilayer printed circuit board, when the innermost conductor circuit is formed by the subtractive process, concave portion is caused between the conductor circuits formed by the etching treatment. Therefore, when the resin insulating layer is formed on the wiring substrate having the concave portion between the conductor circuits, as shown in FIG. 4(a), the surface of the resin insulating layer becomes uneven and hence there is caused a problem that the viahole shape formed in the resin insulating layer and junction pad are deformed to cause poor mounting.
Concretely, the resin insulating layer formed in a large area portion of the conductor circuit becomes thicker as compared with the resin insulating layer formed in a small area portion of the conductor circuit. As a result, the exposure and development conditions of the viahole vary in accordance with the thickness of the resin insulating layer, so that
1. When the exposure and development conditions are matched with the thin portion of the resin insulating layer, a developing residue is created in the viahole formed in the thick portion of the resin insulating layer to close the opening of the viahole and hence the connection to a wiring layer arranged on the upper layer becomes incomplete (see FIG. 4(b)).
2. When the exposure and development conditions are matched with the thick portion of the resin insulating layer, the insulating material is peeled from the vicinity of the viahole formed in the thin portion of the resin insulating layer, which results in the poor insulation (see FIG. 4(c)).
On the other hand, the resin insulating layer being inevitable in the above additive process is generally formed by simply applying one kind of the resin composition. However, as the printed circuit board is recently used in various fields, it is very difficult to form the resin insulating layer so as to exhibit excellent application adaptability in all fields. Lately, the resin insulating layer constituting the printed circuit board tends to become thinner in accordance with the high performance and high densification of electric parts, which is difficult to ensure the insulating property between the conductor layer and another conductor layer and hence results in the lowering of reliability in the multilayer printed circuit board.
Furthermore, the surface of the resin insulating layer is roughened by using an acid or an oxidizing agent and dissolving and removing particles of heat-resistant resin soluble in the oxidizing agent or the like, which are dispersed in heat-resistant resin hardly soluble in the oxidizing agent or the like. However, if aggregation or the like is caused in the heat-resistant resin particles to be dissolved and removed, the depth of the roughened layer is not constant. Particularly, when the resin insulating layer is thin, the depth of the roughened layer may arrive at the thickness of the resin insulating layer and hence the interlaminar insulation property naturally required in the resin insulating layer considerably lowers.
It is a main object of the invention to provide a new structure of a resin insulating layer capable of providing a multilayer printed circuit board having an excellent interlaminar insulation property without lowering the peel strength even if the thickness of the resin insulating layer is thin.
It is another object of the invention to provide a multilayer printed circuit board having an excellent resolution capable of conducting exposure and development treatments under same conditions without forming unevenness on the surface even if an innermost layer of conductor circuits is formed by an etching treatment through the subtractive process as well as a method of producing the same.
It is the other object of the invention to provide a multilayer printed circuit board having excellent resolution, interlaminar insulation property and resistance to cool and heat shocks.
It is a still further object to provide a resin filler for smoothening the surface of the substrate suitable for the multilayer printed circuit board.